Process for producing microporous tubes of polytetrafluoroethylene

ABSTRACT

Polytetrafluoroethylene resin tubes having small pore size and high porosity are produced by a process which includes stretching the tubes by drawing using a metal die and a metal plug. By using the die and plug, the stress necessary for stretching is converted to a compressive force in the thickness direction of the tube. This compressive force is an essential factor in producing the microporous structure of the tubes. In addition, when stretching is performed by drawing, a ratio of thickness to inside diameter of the tube of 0.07 or less can be easily achieved.

This is a Division of application Ser. No. 832,267, filed Sept. 12, 1977now U.S. Pat. No. 4,177,334.

BACKGROUND OF THE INVENTION

The present invention relates to polytetrafluoroethylene resin tubeshaving small pore size simultaneously with a high porosity and to aprocess for producing the same.

Although nonporous tubes or pipes composed of polytetrafluoroethyleneresin having various inside diameters and wall thicknesses have been onthe market, only quite limited kinds of porous tubes or pipes have beenproduced. Processes for producing porous polytetrafluoroethylene resintubes are described in Japanese Patent Publications Nos. 13560/67 and18991/76. The process for producing porous polytetrafluoroethylene resintubes as described in these patent publications comprises molding apolytetrafluoroethylene resin by a paste method to form a tube andheating it to 327° C. or more in a stretched state. The tubes producedby such processes have a microstructure comprising nodules linked to oneanother by fine filaments, and the space surrounded by the filaments andthe nodules defines a pore. As a result of various studies about tubemolding, the present inventors have found that prior art moldings onlyhaving comparatively large pore size as large as 1μ or more aregenerally obtained by these known processes and they have a defect thatporosity remarkably decreases when the pore size is 0.5μ or less.

On the one hand, in spite of being nonporous or porous, there is adefinite relation between the inside diameter and the thickness of thepolytetrafluoroethylene resin tubes. Namely, if the inside diameterincreases, the thickness has a tendency to increase and the ratio of thethickness to the inside diameter is generally larger than 0.1. Thisdepends upon the method of producing the polytetrafluoroethylene resin,such as the paste extruding method, etc. The reasons for this are thatthe tube before sintering at about 327° C. or more is very brittle andthe tube is more easily broken by a slight exterior force if the insidediameter becomes larger. For use in filtration or separation of amixture such as solids, liquids and gases, etc., it is often preferredto have a small thickness irrespective of the inside diameter of thetube.

SUMMARY OF THE INVENTION

The first object of the present invention is to providepolytetrafluoroethylene resin tubes having small pore size and highporosity.

The second object of the present invention is to provide porouspolytetrafluoroethylene resin tubes having small thickness such that aratio of thickness to inside diameter is 0.07 or less and having a highporosity.

The third object of the present invention to provide a process forexpanding the produced microporous tubes.

In the present invention, when the tube is stretched in the lengthwisedirection, it is stretched by drawing using a metal die and a metalplug. In this way, the stress necessary to stretch the tube is convertedinto a compressive force by the plug acting against the die. Ifnecessary, the tube so produced is expanded by reducing the pressure onthe outside thereof simultaneously with sintering at 327° C. or more.Thus, a microporous tube is obtained.

BRIEF DESCRIPTION OF THE DRAWING

In the drawings:

FIG. 1 is a diagram which shows a characteristic range A, B, C and D ofthe porosity and the bubble point obtained by stretching by drawing anda characteristic range in the prior case of not stretching by drawing;and

FIG. 2 is a simplified cross-sectional view which shows the relation ofthe plug 1, the tube 2 and the die 3 suitable for stretching by drawing.

DETAILED DESCRIPTION OF THE INVENTION

The moldings of the present invention will be illustrated in detail bycomparison with the prior moldings. The characteristics of thesemoldings are shown by porosity, pore size, thickness, inside diameterand strength, etc. Among these, the porosity and the pore size are themost important characteristics of the porous moldings. The porosity canbe calculated by measurement of the specific gravity in air and that inwater. Further, although an indication of the pore size differsdepending on the method of measurement, the pore size used in thisspecification means the maximum pore size. The maximum pore size can becalculated by measurement of the bubble point, namely, the pressure atwhich the first bubble is generated when the molding is wetted by aliquid having a low surface tension such as alcohol, etc. and airpressure is applied to one face of the wetted molding while graduallyincreasing the air pressure. Refer to, for example, ASTM F316-70. Thebubble point is in inverse proportion to the maximum pore size. Thelarger the bubble point is, the smaller the maximum pore size is.

Moldings obtained by the prior art method which comprises stretching atube while heating have considerably large pore size, although the poresize somewhat differs by stretching conditions. Namely, in the prior artmethod, only moldings having a low bubble point are obtained. If it isintended to increase the bubble point by reducing the stretching ratio,the moldings tend to have a low porosity. More specifically, in order tomake the porosity 80% or more, the bubble point becomes 0.2 kg/cm² orless. On the other hand, in order to make the bubble point 0.5 kg/cm²,moldings having a porosity of 60% or less are obtained.

In contrast, in moldings of the present invention, bubble points ofbetween 0.26 kg/cm² -0.8 kg/cm² can be suitably obtained while keepingthe porosity 80% or more. Further, even if the bubble point is 1.0kg/cm², moldings having a porosity up to 75% can be obtained. Theserelations are shown in detail in FIG. 1. As can be understood from thisfigure, it is possible to obtain moldings having porosities between20-60% and bubble points between 1.5 kg/cm² or 2.0 kg/cm². This range isshown as an area surrounded by straight lines linking each point A, B, Cand D. The porosity and the bubble point at each point are as follows:Point A (96%, 0.02 kg/cm²), Point B (54%, 2.0 kg/cm²), Point C (20%, 2.0kg/cm²) and Point D (30%, 1.0 kg/cm²).

In the following, a process for producing the moldings of the presentinvention is illustrated. The first step is to obtain a tubular moldingby the paste method. In this step, almost all resins can be used if theyare made as a fine powder. As the liquid lubricating agent for mixing,materials which are capable of wetting the surfaces of the resin andcapable of being removed by evaporation or extraction at a temperaturebelow the decomposition point of the resin can be used. Thepolytetrafluoroethylene resin composition containing the liquidlubricating agent is molded by extrusion by means of a ram type extruderto form a tube. Then, the liquid lubricating agent is removed from thetube by evaporation or extraction. These steps are the same as or nearlysimilar to the steps of molding tubes in the piror art paste method.However, next a stretching step is performed and this is different fromthe known method. This is a distinguishing feature of the presentinvention.

In stretching the tube, it is necessary to provide an exterior tensilestress higher than the tensile strength of the tube. Hitherto, fixedpoints or fulcrums for stress loading are provided in the lengthwisedirection of the tube. For example, one end is fixed and the other endis drawn. A similar effect can be accomplished by stretching by means ofrolls equipped with grooves and having a different revolving ratio. Inthe present invention, fixed points or fulcrums for stress loading areprovided in the lengthwise direction and a reduction die and plug isprovided through the thickness of the tube so as to disperse a part ofthe stress in the diameter direction or the thickness direction of thetube. Thus, the reduction die and plug functions as a fixed point or afulcrum for the stress load in the thickness direction of the tube.

FIG. 2 illustrates the stretching process according to the invention.The plug 1 functions as a mandrel and is composed of two columnar parts,one having a size smaller than the initial inside diameter of the tube 2and a tappered part. On the other hand, the die 3 is composed of anorifice part having a size smaller than the initial outside diameter ofthe tube 2 and a tapered part. It is preferred that the angle of thetapered part in the die 3 be larger than that in the plug 1.

When the tube 2 is stretched in the direction of the arrow a part of thestress is converted into a compressive force in the thickness directionin the tapered parts of the plug 1 and the die 3, and consequently thethickness of the tube 2 becomes small. Of course, in this case, the plug1, the tube 2 and the die 3 are all heated to a temperature lower thanthe melting point of the polytetrafluoroethylene resin. Since theresidual liquid lubricating agent often evaporates, it is preferred toprovide openings for dispersing the evaporated gas in the center of theplug 1. The compressive force depends upon the angle of the taperedparts of the plug 1 and the die 3, the inside diameter of the orifice inthe die 3, the outside diameter and weight of the plug 1, the thicknessof the tube 2 and the temperature, speed or stretching ratio forstretching by drawing. In order to obtain small pore size and a highporosity, it is preferred that conventional stretching which does notuse the die and plug be first carried out and then stretching by drawingis carried out using the die and plug.

In carrying out stretching without the use the die and plug, an increaseof the porosity and an increase of pore size first occur. Then, the poresize decreases; that is, the bubble point increases by carrying outstretching by drawing, while the porosity somewhat decreases. In casesof carrying out stretching only by drawing, the porosity does notsufficiently increase even though the pore size decreases or, on thecontrary, the pore size sometimes does not decrease when the porosity islarge. Further, if the stretching ratio is too large, the pore sizeincreases again as a function of the compressive force. Therefore, it ispreferred to generate the compressive force at a comparatively lowstretching ratio from the viewpoint of producing a tube having a highporosity and small pore size.

The molding stretched by drawing is then sintered at a temperature ofabout 327° C. or more to fix its structure obtained by stretching. Inthis case, the thickness of the tube becomes thin and the ratio ofthickness to inside diameter of the tube is nearly the same as the caseof an extrusion tube prepared using the paste method. However, if thetube is held so that it is not contracted in the lengthwise direction ofthe tube and is expanded in the diameter direction by reducing thepressure of the outside of the tube when sintering at a temperature ofabout 327° C. or more is carried out, the thickness of the tube becomeseven thinner and the inside diameter thereof becomes larger.Consequently, it is possible to obtain a ratio of thickness to insidediameter of 0.07 or less and sometimes 0.03. Such porous tubes having aratio of 0.03 generally can not be obtained by the paste extrusionmethod, and long moldings can not be obtained even if short moldings canbe produced.

It has been found that the degree of expansion in the diameter directioncan be quite freely selected by varying the reduction in outsidepressure. However, if the degree of expansion is large, though theporosity increases, the bubble point tends to decrease as illustrated bythe characteristic in the upper left area of FIG. 1.Polytetrafluoroethylene resin tubes having such small pore sizesimultaneously with a high porosity did not exist hitherto, although ithas been known that such tubes would provide meaningful materials forartificial tubular internal organs or industrial filtration.

In the following, the present invention will be illustrated withreference to examples. However the scope of the present invention is notlimited to these examples.

EXAMPLE 1

3 kg of Polyflon F-104 (tradename of polytetrafluroethylene fine powderproduced by Daikin Kogyo Co.) was mixed with 0.75 kg of Deo-Base(tradename of a light petroleum distilate produced by Witco ChemicalCo.) and the mixture was extruded at a section reduction ratio of 580 toform a tube having a 5.5 mm outside diameter and a 4.0 mm insidediameter. After removing the Deo-Base by extraction withtrichloroethylene, the tube was stretched fourfold at 300° C. in thelengthwise direction at a 20 cm/min tube supplying rate. The tube wasthen stretched twofold or threefold by drawing using a die, having a 4.2mm orifice diameter and a 30° taper angle, and a plug, having columnarparts with outside diameters of a 4.0 mm and 3.0 mm and a 20° taperangle, at a 300° C. tube temperature in the lengthwise direction at a 20cm/min tube supplying rate.

The tube stretched by drawing was sintered by passing the tube throughan electric furnace at 510° C. at 60 cm/min. The results are shown inTable 1. For comparison, the characteristics of a tube produced underthe same conditions without using the die and the orifice are also shownin Table 1.

                  TABLE 1                                                         ______________________________________                                              Second                         Inside                                   Exper-                                                                              stret-                  Bubble diam- Thick-                             iment ching   Die and  Porosity                                                                             point  eter  ness                               No.   ratio   plug     (%)    (kg/cm.sup.2)                                                                        (mm)  (mm)                               ______________________________________                                        1     2 times Present  40.4   1.3    3.2   0.195                              2     3       Present  81.6   0.31   3.3   0.346                              3     2       Absent   76.0   0.20   3.4   0.48                               4     3       Absent   85.3   0.11   3.4   0.44                               ______________________________________                                    

In the tubes which were not stretched by drawing, the ratio of thicknessto inside diameter is more than 0.1. On the other hand, it is clear thatthe bubble point is high in cases of tubes which were produced bystretching using the die and the plug.

EXAMPLE 2

The tube stretched by drawing in Example 1 was introduced under reducedpressure in an electric furnace and heated to 327° C. or more while thetube was expanded by reducing the pressure on the outside of the tube.The characteristics obtained are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Exper- Second            Bubble Inside                                        iment  stretch- Porosity point  diameter                                                                             Thickness                              No.    ing ratio                                                                              (%)      (kg/cm.sup.2)                                                                        (mm)   (mm)                                   ______________________________________                                        5      2 times  56.4     0.76   5.8    0.12                                   6      3        83.7     0.27   5.6    0.28                                   ______________________________________                                    

The bubble point decreases as compared with the cases in Example 1, butthe porosity tends to increase and the ratio of thickness to insidediameter becomes small, which is 0.02 and 0.05.

EXAMPLE 3

The same experiment was carried out as in Example 1 except that thecolumnar parts of the plug had outside diameters of 4.0 mm and 3.5 mmand the taper angle was 17°, 21° or 36°. The first stretching wascarried out by stretching threefold and the second stretching wascarried out by stretching twofold or threefold.

The sintering was carried out without using a reduced pressure.

                  TABLE 3                                                         ______________________________________                                                      Second                                                          Exper-        stretch-       Bubble Inside Thick-                             iment Taper   ing     Porosity                                                                             point  diameter                                                                             ness                               No.   angle   ratio   (%)    (kg/cm.sup.2)                                                                        (mm)   (mm)                               ______________________________________                                         7      17°                                                                          2 times 61.2   1.12   3.4    0.27                                8    17      3       73.7   0.43   3.4    0.18                                9    21      2       60.3   1.55   3.5    0.21                               10    21      3       70.5   0.46   3.4    0.18                               11    36      2       57.4   1.76   3.6    0.21                               12    36      3       68.7   0.54   3.4    0.18                               ______________________________________                                    

When the taper angle increases, the porosity slightly decreases andconsequently the bubble point increases and the thickness decreases.This is supposed to be due to a change of compressive force in thethickness direction.

EXAMPLE 4

A tube having an outside diameter of 8.0 mm and an inside diameter of6.0 mm was produced in the same manner as in Example 1. After the tubewas stretched threefold, it was stretched twofold by drawing using a diehaving a 5.1 mm orifice diameter and a 30° taper angle and a plug havingcolumnar parts with outside diameters of 6.0 mm and 5.1 mm and a 25°taper angle at a 290° C. tube temperature at a 25 cm/min tube supplyingrate. The tube sintered at 360° C. had a 68% porosity, a 0.62 kg/cm²bubble point, a 4.8 mm inside diameter and a 0.23 mm thickness. The tubewhich was expanded by reducing the outside pressure while sintering at360° C. had a 85% porosity, a 0.39 kg/cm² bubble point, a 9.5 mm insidediameter and a 0.09 mm thickness.

What is claimed is:
 1. A process for producing microporous tubes whichcomprisesmolding a polytetrafluoroethylene resin containing a liquidlubricating agent by a paste method to form a tube, removing the liquidlubricating agent, stretching by drawing the tube in a lengthwisedirection using a die and a plug, and sintering at a temperature ofabout 327° C. or more whereby as a result of said stretching and saiddrawing, a microporous tube is produced having a porosity and a bubblepoint in a range between the point A, B, C and D defined in thefollowing table;

    ______________________________________                                                  porosity   bubble point                                             ______________________________________                                        A           96%          0.02 kg/cm.sup.2                                     B           54%          2.0 kg/cm.sup.2                                      C           20%          2.0 kg/cm.sup.2                                      D           30%          1.0 kg/cm.sup.2                                      ______________________________________                                    

and having a ratio of wall thickness to inside diameter of about 0.1 orless.
 2. A process for producing microporous tubes as set forth in claim1, wherein the stretching in the lengthwise direction is carried out twoor more times and the final stretching is carried out by drawing usingthe die and the plug.
 3. A process for producing microporous tubes asset forth in claim 1 which comprises sintering the tube at about 327° C.or more simultaneously with expanding the tube by reducing the pressureon the outside of the tube after carrying out stretching by drawing.